In relatively recent years, fiber reinforced composite parts have replaced parts and components previously made of structural metals, such as steel, aluminum and the like. This is particularly true in the aircraft industry, where it is recognized that the reinforced composite components can provide at least the same degree of strength and structural integrity as conventional materials, and oftentimes with significantly less weight and cost. Many of the reinforced components are made with reinforced cloth, which is ultimately formed to a desired shape, impregnated with a hardenable resin and cured to form a rigid structure.
The advantages achieved by stitching, such as, for example, loop stitching through fiber reinforced composite components for reinforcing two abutting components in a secured position and to resist delamination and peel forces, has been well established. See, for example, U.S. Pat. No. 4,256,790 to Lackman, et al., which teaches the sewing of a series of stitches through the thickness of panels while they are in a staged condition, which allows the two panels to be co-cured as assembled and form a strong reinforced composite structure. U.S. Pat. No. 4,206,895 to Olez, also discloses the strengthening of a joint in a bonded fiber structure of two or more fiber reinforced components by the use of high strength threads inserted through the joint.
Obtaining a three-dimensional reinforcement in a tight, restricted area, as, for example, between stiffeners and intercostals of a stiffened skin structure, is difficult, and in many cases, virtually impossible with conventional stitching machines. The use of stitching, as, for example, shown in the Olez U.S. Pat. No. 4,206,895, and in the Lackman, et al. U.S. Pat. No. 4,256,790, has improved damage tolerance and resistance to delamination, peel and tensile pull-off loads. Moreover, this three-dimensional stitching has been found to be effective in reducing in-plane shear in laminated structures.
Due to the size and construction of conventional stitching machines, however, the location and the overall size of the bobbins (or loopers, which are used in chain stitching) greatly restrict the area in which stitching may be accomplished. Consequently, in many areas where three-dimensional reinforcement would be desired, conventional automatic stitching is not an option because of the difficulty, if not impossibility, of operating the stitching machines in confined and restricted areas.
Tufting has also been considered as a means of three-dimensional reinforcement in confined and restricted areas. However, tufting is limited in that it does not retain the compaction of the dry fiber preforms which is achieved by conventional stitching which is not tufted.
Stapling, although having some advantages, is usually performed with staples formed of metal, and metal in certain cases may be unacceptable. As a simple example, metal components are electrically conductive, and electrical conductivity may be undesirable in certain applications. Effective thermoplastic staples are not yet available. There have even been attempts to make individual pins from carbon fibers and a thermoplastic resin, which are extruded. However, these pins are very costly to make, and furthermore, they are quite labor-intensive when installed, even though they may improve the desired properties of the components in which they are used.
U.S. Pat. No. 4,528,051 to Heinze, et al. discloses a method for strengthening fiber reinforced components using a multitude of metal or synthetic material pins driven into the layers of fibers and resin of adjacent structural components. The resin is then cured with the pins in place to improve the strength of the structural components and to prevent the peeling of one layer away from an adjacent layer. In certain embodiments, the pins are flexible and formed of Kevlar.RTM. thread. A stitching mechanism or sewing machine is used to insert the threads. As such, these embodiments suffer from the disadvantage noted above; namely, that the stitching operation cannot be performed in confined and restricted areas.
Other embodiments of the Heinze patent use stiff metal pins inserted in a pin carrier in the form a belt or webbing. The webbing may be bonded to one of the fiber reinforced composite components after the metal pins have been inserted. However, the use of conductive metal components can be disadvantageous as noted above. In addition, the highly flexible nature of the webbing can make the process of inserting the pins, unless applied using the illustrated roller apparatus, cumbersome and manually intensive.
Accordingly, there has been a need for a reinforcing member which can be used for securing two or more reinforced components together in restricted and confined areas. More specifically, there has been a need for a reinforcing member which can be used for securing intercostals and stiffeners ("T" stiffeners in 0.degree., 90.degree. or other orientations relative to one another) to a reinforced plastic skin, particularly in confined and limited spaces. Such a reinforcing member would preferably be non-conductive and be capable of being fairly easily integrated into the reinforced composite structure.